Abstract: For the safety evaluation of narrowband used in engineering structures, the propagation characteristics of guided waves in a narrowband are investigated experimentally. Firstly, the characteristics of guided waves propagated in an infinite plate is theoretical analyzed; then the contrastive experiments are conducted in narrowbands with different widths using different modes of guided waves. It is found that comparing to the lamb modes, shear horizontal waves (SH0) are almost not affected by the boundary of a narrowband, and the width variation of a narrowband has no influence on the waveforms of an SH0 mode when the width of a narrowband is bigger than the width of electromagnetic acoustic transducers. Finally, the SH0 mode guided wave is used to the non-destructive testing (NDT) of flat steel in a ground grid. It is showed that a SH0 mode can propagate a long distance in flat steel, and can be used for NDT of buried flat steel. The developed method provides feasible scheme for the health monitoring of narrowbands in engineering structures.